This invention relates to image-receiving elements for photographic products and processes. More particularly, it relates to image-receiving elements especially suited to application in photographic diffusion transfer products and processes.
Diffusion transfer photographic products and processes have been described in numerous patents, including, for example, U.S. Pat. Nos. 2,983,606; 3,345,163; 3,415,644; 3,594,164; 3,594,165 and 3,647,437. The aforementioned U.S. Pat. No. 3,415,644 (issued Dec. 10, 1958 to Edwin H. Land) discloses photographic products and processes wherein a photosensitive element and an image-receiving element are maintained in fixed relationship prior to exposure, and wherein this relationship is maintained as a laminate after processing and image formation. In these products and processes, the final image is viewed through a transparent (support) element against a reflection, i.e., white, background. Photoexposure is made through said transparent element and application of the processing composition provides a layer of light-reflecting material to provide a white background for viewing the final image through said transparent support. The light-reflecting material (referred to in said patent as an "opacifying agent") is preferably titanium dioxide, and it also performs an opacifying function, i.e., it is effective to mask the developed silver halide emulsions so that the transfer image may be viewed without interference therefrom, and it also helps to protect the photoexposed silver halide emulsions from postexposure fogging by light passing through said transparent layer if the photoexposed film unit is removed from the camera before image-formation is completed.
In the aforementioned U.S. Pat. No. 3,647,437 (issued Mar. 7, 1972 to Edwin H. Land), there are disclosed photographic products which may be processed outside of the camera in which the film is exposed, fogging of the film by ambient light being prevented by provision of one or more opacifying dyes, sometimes referred to as light-absorbing optical filter agents, appropriately positioned in the film unit after photoexposure. According to a particularly useful embodiment of the invention there described, the film unit is a film unit of the type described in the aforementioned U.S. Pat. No. 3,415,644 and comprises first and second sheet-like elements, the first sheet-like element comprising an opaque base carrying a silver halide emulsion, and the second sheet-like element comprising a transparent support carrying an image layer, i.e., a layer adapted to receive an imagewise distribution of an image-forming material initially present in said first sheet-like element. After photoexposure a processing composition, adapted to develop the exposed silver halide emulsion and to form the desired image in said image layer, is distributed in a thin layer between said sheet-like elements. The processing composition contains a light-reflecting pigment, such as titanium dioxide, and at least one light-absorbing optical filter agent, such as a pH-sensitive phthalein dye which is colored at the initial pH of said processing composition. As is disclosed in the patent (aforementioned U.S. Pat. No. 3,647,437), the concentrations of light-reflecting pigment and optical filter agent(s) are such that the layer of processing composition is sufficiently opaque to light actinic to the silver halide emulsion that the film unit may be ejected from the camera immediately after the processing composition is distributed, notwithstanding the fact that the second sheet-like element will transmit light incident on the surface thereof. This opacification system is quite effective and is employed in Polaroid Land SX-70 film. The light-absorbing capacity of the optical filter agent is discharged after this ability is no longer needed, so that the optical filter agent need not be removed from the film unit. Where the optical filter agent is a pH-sensitive dye, such as a phthalein indicator dye, it may be discharged or decolorized by reducing the pH after a predetermined time, e.g., by making available an acid-reacting material such as a polymeric acid.
In the preferred embodiments of the opacification system described in U.S. Pat. No. 3,647,437 the concentrations of the light-reflecting pigment and light-absorbing optical filter agent in the layer of processing composition will be such that that layer will have a transmission density of at least about 6 but a reflection density not greater than about 1. The presence of a long chain substituent, e.g., a long chain alkoxy group, on the optical filter agent is useful in reducing its diffusibility so that diffusion to the image-receiving layer is minimized.
A reflection density of about 1 will be recognized as very small compared with a transmission density of 6 or more for the same layer. In practice it has been possible to use a concentration of optical filter agents and titanium dioxide such that the reflection density of the processing composition layer, as measured about 30 seconds after distribution, is much lower than 1, e.g., about 0.5 to 0.6. While transferring dye and the emerging dye image may be seen at opacification system reflection densities of about 0.5, the presence of such temporary coloration of the highlight or white areas of the image, and the temporary distortion of the colors of the already transferring image dyes, is aesthetically undesirable.
As noted above, where the optical filter agent is a pH-sensitive dye, it is "discharged", i.e., rendered substantially colorless, by a reduction of the pH of the strata containing the optical filter agent. These strata include the light-reflecting pigment layer, provided by the processing composition, as well as the image-receiving layer and any other layers between the light-reflecting pigment layer and the transparent support through which the final image is viewed. This pH reduction is effected, to a pH level below the pKa of the optical filter agent, after a predetermined time. This delay is necessary in order that silver halide development be substantially completed before incident light is transmitted to the developing silver halide emulsions. Since the image dyes are preferably soluble and diffusible at the initial pH of the process but substantially nondiffusible at a lower pH, reduction of the pH to the appropriate lower pH after a predetermined period serves the very important function of controlling unwanted continued transfer of image dyes after the desired dye image has been formed.
It will be recognized that these desired results of pH reduction are only partly compatible, for early pH reduction to provide a white background early in the process could prematurely stop transfer of image dyes, resulting in a pale, i.e., low density, image which may also have an unbalanced color balance.
In U.S. Pat. No. 4,294,907 (issued Oct. 13, 1981 to I. Y. Bronstein-Bonte et al.), and in U.S. Pat. No. 4,298,674 (issued Nov. 3, 1981 to Edwin H. Land et al.), there are disclosed photographic diffusion transfer products and processes of the foregoing type where the background appears substantially white to the viewer, substantially immediately after the processing composition is applied while retaining opacification. As is disclosed in these patents, the reflection density provided by the layer of processing composition (containing the light-reflecting pigment and the optical filter agent) can be reduced without significantly reducing the transmission density thereof. This highly desirable improvement is obtained by the provision of a polymeric "decolorizing" layer between the image-receiving layer and the layer of processing composition. The decolorizing layer comprises a relatively thin layer of a substantially non-diffusible polymeric agent adapted to decolorize the small concentration of optical filter agent present immediately adjacent the interface between the processing composition and the decolorizing layer. As a consequence of the decolorization of optical filter agent at this interface, image formation appears to emerge rapidly from a substantially "white" background.
While the utilization of a polymeric decolorizing agent in accordance with teachings and embodiments of the aforesaid patents permits one to effectively increase the apparent whiteness of the layer of the processing composition against which the image is viewed, without reducing the transmission density of the "white" layer to any significant extent, the provision of such decolorizing layer involves the handling and application of the polymeric decolorizing material as a separate layer over an image-receiving layer. Thus, a solvent system for the decolorizing polymer, selected to avoid incompatibility with the image-receiving layer or other layer of an image-receiving element, is utilized for the application of the decolorizing layer over a suitable image-receiving layer. The decolorizing layer, depending upon the particular nature thereof, may be more or less adherent to the particular image-receiving layer utilized.